1. Field of the Invention
This invention relates to a metallic mirror and a metallic rotary polygonal mirror which are usable as reflecting mirrors in copying machines, facsimile machines, laser beam printers or the like and as mirrors for microchip optical scanners, and also relates to a process for their production. More particularly, this invention relates to a metallic mirror and a metallic rotary polygonal mirror which have a superior environmental stability and a stability in reflectance, having a high reflectance, and a process for their production.
2. Related Background Art
Conventionally available metallic rotary polygonal mirrors include, e.g., the following (1), (2) and (3).                (1) A metallic mirror whose rotary polygonal mirror substrate made of aluminum or an aluminum alloy has been anodized on its cut mirror surface to form a transparent film (anodic oxide film) to provide a mirror-surface protective film (Japanese Patent Application Laid-Open No. 58-184903).        (2) A metallic mirror whose rotary polygonal mirror substrate made of aluminum or an aluminum alloy is provided with a metallic thin film of Cr formed by vacuum deposition and further provided thereon with a high-reflectance metallic thin film of Cu formed by vacuum deposition (Japanese Patent Application Laid-Open No. 60-195502).        (3) A metallic mirror whose rotary polygonal mirror substrate made of aluminum or an aluminum alloy is provided with an intermediate layer, a metallic reflective layer and a protective film which are formed by vacuum deposition in order; the intermediate layer being a Cr layer having a layer thickness of from 50 nm to 100 nm, the metallic reflective layer a Cu layer having a layer thickness of from 100 nm to 150 nm, and the protective film an Al2O3 layer having a layer thickness of from 150 nm to 200 nm (Japanese Patent Application Laid-Open No. 6-208076).        
Of the above conventional metallic mirrors, the mirror (1) has an average reflectance of about 85%, which cannot be said to be a sufficient reflectance for the achievement of higher speed in copying machines and laser beam printers. Also, when the anodic oxide film is formed on the substrate made of aluminum or an aluminum alloy, the presence of any impurities such as Si on the rotary polygonal mirror substrate may possibly cause pit-like defects in the anodic oxide film. Accordingly, the aluminum or aluminum alloy must be those having a high purity, and hence expensive materials are required for the rotary polygonal mirror substrate, resulting in a high cost.
As for the mirrors (2) and (3), they have a high reflectance, but have an environmental stability problem. When these metallic rotary polygonal mirrors are placed in a high-temperature and high-humidity environment (e.g., 45° C., 95% RH, 100 hrs, O3 1 ppm; or 70° C., 85% RH, 100 hrs, O3 1 ppm), the following phenomena may take place.                i) As shown diagrammatically in FIG. 3, a dissolution reaction of a metal and a reduction reaction of an oxygen take place, which are considered due to the formation of local cells. What makes this reaction take place is the water-absorptive dust adhered to the film surface, where H2O locally enters the film. Oxygen dissolved in the water is further involved, and the water passes from the outermost layer through SiO2→Al2O3→Cu→Cr to reach the underlying substrate Al. As the result, an aluminum oxide is formed such as Al2O3, which causes volume expansion to increase film lift, resulting in defects. This phenomenon can be explained by oxidation reduction reaction as shown by the following scheme.Anode: Al→Al3+3e−(dissolution reaction of Al)Cathode: O2+2H2O+4e−→4OH (reduction reaction of oxygen)        
Optical-microscope photographs and a cross-sectional photograph [analysis by FIB (focused ion beam)] of such a corrosion phenomenon are shown in FIGS. 6 and 7 and FIG. 8, respectively.
More specifically, the present inventors placed in the high-temperature and high-humidity environment (45° C., 95% RH, 100 hrs, O31 ppm; or 70° C., 85% RH, 100 hrs, O3 1 ppm) a metallic rotary polygonal mirror comprised of an aluminum substrate, and a Cr layer as an intermediate layer, a Cu layer as a reflective layer and an Al2O3 layer (dielectric layer) and an SiO2 layer as protective layers superposed on the aluminum substrate, and observed it. As a result, as shown in FIGS. 6 and 7 (both optical-microscope photographs at corroded portions) and FIG. 8 (an FIB cross-sectional photograph), corrosion was found to have occurred.                ii) A phenomenon of “film lifting” may occur as shown in a photograph 4 given as FIG. 9 and photographs 6 to 8 given as FIGS. 10 to 12. As can be seen from these cross-sectional photographs, films are separated at the interface between the intermediate layer Cr film and the reflective layer Cu film.        
Measurements of each film stress between the intermediate layer Cr, the reflective layer Cu and the dielectric layer Al2O3 reveal that the Cr film has a stress of +4.15×108 Pa, the Cu film +1.53×108 Pa, and the Al2O3 film −0.63×108 Pa, showing great differences in stress among them, which can be presumed to be caused by the separation between the Cr film and the Cu film.